1. Field of the Invention
The present invention relates to a phase shifting mask and a method of manufacturing the same. The mask produced by the present invention is adapted for use in forming a variety of patterns, such as a resist pattern for example in the process of manufacturing a semiconductor device. And the present invention is capable of minimizing the number of required steps in manufacture of such a phase shifting mask.
2. Description of the Prior Art
In semiconductor devices and so forth, the processing dimensions have tended to become extremely smaller year after year. Relative to the technology of photo-lithography employed for manufacture of such miniaturized semiconductor devices, there is known, as a means for further enhancing the resolution, a phase shifting technique which causes a phase difference in the light transmitted through a mask, thereby improving the light intensity profile.
The phase shifting method is disclosed in Japanese Patent Laid-open No. Sho 58 (1983)-173744; Marc D. Levenson et al., "Improving Resolution in Photolithography with a Phase Shifting Mask", IEEE Transactions on Electron Devices, Vol. ED-29, No. 12, Dec. 1982, pp. 1828-1836; and Marc D. Levenson et al., "The Phase Shifting Mask II; Imaging Simulations and Submicrometer Resist Exposures", Ditto, Vol. ED-31, No. 6, Jun. 1984, pp. 753-763.
The conventional phase shifting method known heretofore will now be described below with reference to FIG. 1(a). In an exemplary case of forming a line-and-space pattern, it is usual that, as shown in FIG. 1 (a), a light shielding region 10 is formed by the use of a light shielding material such as chromium on a transparent substrate 1 of quartz or the like, and an arrangement of repeated line-and-space patterns is formed to produce an exposure mask. The intensity distribution of the light transmitted through such an exposure mask is represented by a curve A1 in FIG. 1 (a), wherein the intensity is zero in the light shielding region 10 while the light is transmitted through the other regions (light transmitting segments 12a, 12b). Viewing one light transmitting segment 12a as an example, the intensity of the light transmitted therethrough and irradiated to a work member to be exposed is distributed as represented by a curve A2 in FIG. (a), wherein hill-like maximums existent at the feet on both sides due to the diffraction of the light and so forth. The light passed through the light transmitting segment 12b is represented by a one-dot chained line. When the light rays obtained through the individual transmitting segments 12a, 12b are mutually combined, the light intensity distribution is deprived of its sharpness as indicated by a curve A3 to consequently blur the image due to diffraction of the light, hence failing in a sharp exposure. In contrast therewith, if phase shifting films 11a are provided on the light transmitting segments 12a, 12b of the repeated patterns in alternate fashion as shown in FIG. 1 (b), any blur of the image resulting from diffraction of the light is eliminated by inversion of the phase to eventually achieve transfer of a sharp image, thereby improving the resolution and the focusing allowance. More particularly, when a phase shifting film 11a is formed on the light transmitting segment 12a as shown in FIG. 1 (b) in such a manner as to cause a phase shift of 180.degree., for example, the light passed through the phase shifting film 11a is inverted as represented by a curve B1. Meanwhile the light obtained through the adjacent light transmitting segment 12a is not passed through the phase shifting film 11a, so that none of such a phase inversion is induced. Therefore, on the work member to be exposed, the mutually phase-inverted light rays cancel each other in the position B2 at the foot of the light intensity distribution, whereby the distribution of the light irradiated to the work member is rendered ideally sharp, as represented by a curve B3 in FIG. 1 (b).
In the example mentioned, the greatest advantage is attainable by causing a 180.degree. inversion of the phase to ensure the above-described effect. However, for realizing such a satisfactory result, it is necessary for the phase shifting film 11a to have an adequate thickness ##EQU1## (where n denotes the refractive index of the phase shifting film, and .lambda. denotes the wavelength of the exposure light).
In the process of forming a pattern by an exposure, it is customary that a member used for reduced-size projection is termed a reticle, and a member for life-size projection is termed a mask; or a member corresponding to an original sheet is termed a reticle, and a member obtained by duplicating such a reticle is termed a mask. In the present invention, any of the masks and reticles classified by such various definitions is termed a mask in general.
Although the technique utilizing the conventional phase shifting mask mentioned above is remarkably effective for forming an arrangement of repeated patterns such as line-and-space patterns described with reference to FIG. 1, there still exists a problem that such a mask is not easily usable in forming isolated patterns which are not repetitive.
As mentioned, the phase shifting technique causes a phase difference between the light rays for exposure of mutually adjacent patterns and utilizes the effect that the respective light intensities cancel each other. However, since mutually proximate light rays are not existent in the case of forming an isolated line or a contact hole, the above prior art is not directly adapted for realizing the phase shifting technique.
It has therefore been necessary that, as illustrated in FIG. 2 (a), a light transmitting region 12 for transmitting exposure light therethrough without causing any phase shift (phase shift 0.degree.) is provided in conformity with a desired pattern to be formed, and a phase shifting region 11 for causing a phase shift of exposure light (e.g., phase shift 180.degree.) is provided in the proximity of the light transmitting region 12. (Refer to Terasawa et al., Second Draft for Lecture in 49th Applied Physics Society Meeting, Autumn 1988, p. 497, 4a-K-7).
In such prior art, a main space is required for providing a pattern-forming light transmitting segment 12 within a light shielding region 10, and further a subspace is also required for providing phase shifting segments 11. The phase shifting segments 11 are formed along the two sides of and in the proximity of a rectangular light transmitting region 12 which is a main space in a mask of FIG. 2 (a) for forming an isolated line pattern; or the segments 11 are formed along and in the proximity of four sides of a square light transmitting region 12 in a mask of FIG. 2 (b) for forming a hole pattern.
In such prior art, the light transmitting segment 12 and the phase shifting segment 11 need to be spaced apart from each other since any improperly small distance therebetween induces excessive cancellation of the respective light intensities to consequently reduce the transfer pattern. Therefore a problem arises with regard to the necessity of a large area for forming each pattern.
Furthermore, the phase shifting segment 11 needs to have certain dimensions since the effect thereof is diminished in accordance with a dimensional decrease. As a result, there arises another problem that the pattern of the phase shifting segment 11 itself is transferred. FIG. 3 illustrates an exemplary case of forming an isolated space 7 by using the mask of FIG. 2 (a). As shown with some exaggeration, it is unavoidable that a pattern 71 derived from the phase shifting segment 3 is formed.